The present invention relates to reactors for electrical power distribution and transmission systems and, in particular, to a modular construction of a reactor which has a high inductance and a high current carrying capacity.
Shunt reactors are used in conjunction with high voltage, alternating current power transmission and distribution lines to compensate for line charging current and to allow the charge remaining on the line to bleed to ground when the line is opened. Usually, the shunt reactor has an air or nonmagnetic material core or a laminated steel core with air gaps within the coil and the assembly is immersed in oil inside a tank. A shunt reactor construction is disclosed in U.S. Pat. No. 3,362,000 issued to Sealey et al. on Jan. 2, 1968 which discloses a reactor which has an axially short coil and magnetically permeable yoke means adjacent the ends of the coil which straighten the magnetic flux lines within the coil and decrease the length of the average flux path so that substantially all of the reluctance of the magnetic flux path is internal of the coil. A related patent, U.S. Pat. No. 3,362,001 issued to Wishman et al. on Jan. 2, 1968, discloses a shunt reactor wherein the ratio of coil radius to coil axial length is substantially increased in comparison to known reactors of the same voltage rating, thus resulting in an increase in magnetic flux density and a corresponding increase in inductance for a given physical size of reactor. It discloses a shunt reactor wherein the axial spacing between adjacent pancake windings of the reactor coil is substantially reduced in comparison to previously known reactor coils of the same voltage rating, thus permitting reduction in the axial length of the reactor coil and resulting in substantial reduction in size and weight for a given KVA rating in comparison to known reactors at the time. U.S. Pat. No. 3,991,394 issued to Barnwell et al. on Nov. 9, 1976 discloses a power line inductor having a plurality of coaxial coils. The coaxial coils are formed by rectangular conductors wound side by side in a single layer with a selected number of turns. U.S. Pat. No. 2,082,121 issued to Rypinski on June 1, 1937 discloses a time-controlled reactor. It further provides a circuit arrangement for the reactor by which the electromagnetic properties of the reactor may be controlled in accordance with a given time period. It provides a reactor with a plural winding electromagnetic system in which the magnetic effect is controlled by a differential change in resistance in the electromagnetic windings in accordance with a predetermined time cycle. U.S. Pat. No. 3,902,147 issued to Trench on Aug. 26, 1975 discloses an air core duplex reactor consisting of one, two or more sets of two rigid cylindrical assemblies disposed in concentric, radially spaced relation. All of its coils are electrically connected in parallel at one end and have individual connections for the respective sets of coils at the opposite end, one set of coils being interleaved with the other and each coil consisting of a rigid, longitudinally extending sleeve member having a coil wound on a portion of the length thereof extending from adjacent the parallel connected end in a direction toward the opposite end.
One of the major requirements in building high current reactors is the provision of an adequate conductor cross section to carry the required current without overheating. Utilizing a conductor with a large cross sectional area is not a viable solution because such conductors are generally very difficult to handle in the manufacturing process because of their inherent stiffness. Also, with large cross section conductors, losses caused by skin effect and exposure to alternating magnetic fields are increased. Therefore, it is necessary to connect a multiplicity of smaller cables electrically in parallel and provide a means to balance the current in these various parallel paths. An example of this is the design of line trap reactors where several coaxial single-layer coils are electrically connected in parallel with the layer currents being balanced by careful control of the number of turns in each layer. An alternate embodiment of this method utilizes stranded cable as the basic conductor.
The present techniques which utilize parallel layers inevitably lead to low inductance reactors although very high currents can be obtained. For stranded aluminum cable with an area approximately equal to 600 MCM (thousand circular mills) and current rating of 2000 to 3000 amperes, it appears that 1.5 millihenries is about the maximum inductance that can be obtained with the single layer, parallel-coil type of construction. The reason for this is that reactors with larger inductance require more turns per layer and a larger diameter for each layer. This leads to uneconomical reactor proportions when carried to the extreme.
The present invention provides a means for building both high inductance and high current capacity reactors by utilizing a modular design. Since the energy storage capacitor of a reactor is a function of both its inductance and current as defined by E=1/2 LI.sup.2, it should be apparent that, by increasing both inductance L, and current flow I, significant energy storage increases can be obtained. In a reactor made in accordance with the present invention, the required current rating is obtained by connecting several modules electrically in parallel, each of which has the same current rating. The current rating of a particular module is determined by the size of cable which is used. For example, rubber coated 500 MCM aluminum cable may be used which can carry about 250 amperes when wound in a reactor. A 1000 ampere reactor utilizing this conductor would require four modules connected electrically in parallel. To reduce the physical size of the overall reactor, maximum advantage can be taken of the mutual inductance between modules. In a reactor made in accordance with the present invention, all modules are coaxial and may be oriented relative to each other in two distinct ways, axially or radially juxtaposed. In the axially juxtaposed configuration, the generally cylindrical modules of the present invention each have generally identical inside and outside diameters and are positioned in axial relation along the same center line. In this configuration the length of the modules need not necessarily be identical and they will not have the identical number of turns. When the modules are axially associated in this way and the same current is passed through each module, by connecting them electrically in series, the flux linkages with any particular module will depend not only upon its own length and turns but also upon the length and turns of its associated modules comprising the composite reactor. By a judicious choice of module lengths and turns, it is possible to not only obtain identical flux linkages in all the modules, but to preselect its value. The axially juxtaposed configuration also enables the modules to be connected electrically in parallel, with the currents in each module therefore being identical since each module has exactly the same flux linkages.
As mentioned above the modules may also be associated in radial juxtaposition. In this configuration, each module generally has the same length, but different diametric dimensions and number of turns. The modules are both coaxial and concentric. The inside and outside diameters of each module are selected to permit the modules to be associated in a radial relation. As in the case of the axially juxtaposed reactor, described above, the modules must be constructed in such a way that they will be carrying nearly identical currents. Also as in the axially juxtaposed modular reaction described above, the modules may be connected electrically either in series or in parallel.
It is, therefore, an object of the present invention to provide a modular reactor that has a high inductance and a high current carrying capacity, which utilizes modules which may be disposed in a radial or an axial association and may be connected either in series or in parallel.